This invention relates to an amplifier arrangement comprising at least two differential amplifiers having different gain factors and having their inputs and outputs coupled to one another, at least the gain of the differential amplifier having the higher gain factor being variable.
An arrangement of this type, which can be used, for example, for the purpose of the gain control in AM receivers, is known from U.S. Pat. No. 3,909,738. In this known arrangement the differential amplifiers are constituted by emitter-coupled transistor pairs. The base and collector terminals of one pair, which constitute the input and the output of the arrangement respectively, are connected directly to the corresponding terminals of the other pair. The different gain factors are obtained in that the emitter terminals of one transistor pair are connected directly to each other and to a direct current source, whereas the emitter terminals of the other transistor pair are connected to each other and to the common direct current source via negative-feedback resistors of equal resistance value. Therefore, the last-mentioned transistor pair has the lower gain factor. By oppositely varying the direct currents applied to the common emitter terminals of the transistor pairs the overall gain can be varied continuously between this lower gain factor and the gain factor of the other transistor pair.
The input swing in the state in which the input signal is, in fact, only amplified by the amplifier comprising the emitter feedback resistors is dictated by the voltage drop across the negative-feedback resistors in the lines to the emitters. The input swing can be extended by increasing the values of these resistors, but in practice this is in conflict with other requirements.